In the production of nickel phosphorus electroplatings, it has been conventional procedure to use a so-called "Brenner bath" which typically adds nickel in a sulfate form, to provide a part of the constituents of the bath. Brenner's work is reported in his treatise Electrodeposition of Alloys: Principles and Practice, Vol. II, Abner Brenner, Academic Press, 1963, pp. 457-483. There, in Table 35-1, he describes sulfate-containing baths for electrodepositing nickel-phosphorus. It has been found, however, that such "sulfate" baths have relatively poor cathode efficiency, relatively poor bath conductivity (thereby requiring a higher voltage to properly effect plating), and because the sulfate is less soluble than desirable, it forms unwanted precipitates in the bath. Also, baths according to Brenner result in nickel phosphorus platings that have higher tensile stress than desired, less inherent brightness than desired, and a graininess.
According to the present invention, it has been found that all of the above-mentioned deficiencies can be overcome by providing the correct bath constituents, and operating a bath in an appropriate manner to maintain the desired balance between constituents. According to the invention an "all chloride" bath is provided. The bath according to the invention has almost no sulfate. Any sulfate that is present is present in such small quantities that it does not result in the adverse properties set forth above.
The bath according to the present invention includes 0.7-1.3 molar Ni.sup.+, 1-2 molar Cl.sup.-, and 1-3 molar HPO.sub.3.sup.+2. The bath may also include 0.2-0.6 molar PO.sub.4.sup.-3. Some cobalt also may be present, either as a common contaminant of the nickel, or in specified amounts. Typically the bath would have a significantly higher proportion of nickel than cobalt. The bath most typically would be prepared from NiCl.sub.2.6H.sub.2 O and H.sub.3 PO.sub.3, or from Ni(H.sub.2 PO.sub.3).sub.2 and HCl.
The bath according to the present invention has increased cathode efficiency, and conductivity with respect to the Brenner "sulfate" bath, and because the components are more soluble, unwanted precipitates are not in the bath. Further, the nickel phosphorus platings produced from the bath according to the invention have higher inherent brightness, less tensile stress and less "graininess" than "sulfate" baths. One particularly useful bath according to the invention for achieving the desired results comprises: about 1.25 molar H.sub.3 PO.sub.3, about 0.3 molar H.sub.3 PO.sub.4, about 0.9 molar NiCl.sub.2, and about 0.25 molar NiCO.sub.3.
According to the present invention a method is provided for electroplating a nickel phosphorus alloy on a substrate. The method comprises the following steps: (a) Providing a bath comprising 0.7-1.3 molar Ni.sup.+, 1-2 molar Cl.sup.-, and 1-3 molar HPO.sub.3, and minimal amounts of sulfate. (b) Immersing a substrate as a cathode in the bath. (c) Immersing an anode in the bath. And (d) applying an electrical potential across the anode and cathode so as to effect electrodeposition of nickel phosphorus alloy on the substrate, with increased cathode efficiency, increased conductivity, and minimal unwanted precipitates, compared to the results of the use of baths containing significant amounts of sulfate.
In accordance with the teachings of said parent application Ser. No. 732,277 now U.S. Pat. No. 4,673,468 (the disclosure of which is hereby incorporated by reference herein) the phosphoric acid concentration is controlled (kept below 0.5 molar). The bath is kept with an acid titer range of about 9-14, and the anode current density is maintained very high. Typically the anode current density is maintained so that it is always greater than about 200 amperes per square foot. At levels significantly below about 200 amperes per square foot, the desired control of the phosphoric acid buildup and/or free acid concentration does not occur. In fact, anode current densities of at least about 500 amperes per square foot for nickel phosphorus coating baths are preferred. Anode current densities as high as 1250 amperes per square foot are useful, and apparently the upper limits on anode current density are determined by non-electrochemical constraints, such as I.sup.2 R heating, corrosion of accessory electrical components (such as bus bars) at higher voltages, etc.
The anode configuration is chosen so that the desired anode current density is achieved. Typically the anode will be of platinum or rhodium. Anode configurations as in said co-pending application Ser. No. 732,277 now U.S. Pat. No. 4,673,468 are suitable, as are those in co-pending application Ser. No. 609,137 filed May 11, 1984 "Anode Configuration With Nickel Phosphorus Electroplating" (attorney dkt. 11/489). Alternatively, the bath described in U.S. patent application Ser. No. 923,270 filed Oct. 27, 1986, the disclosure of which is incorporated by reference herein, may be used to obtain a more ductile form of the nickel-phosphorus electroplating.
It is the primary object of the present invention to provide an electrochemical bath for electroplating nickel phosphorus alloy onto a substrate that has numerous advantages compared to "sulfate" baths, and to provide nickel phosphorus electroplatings that are advantageous compared to prior art electroplatings. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.